Old English Alphabets in the eighteenth-century books

Old English books or papers from the latter half of the eighteenth century seem to have a different alphabet for 'S', probably Latin, and it seems at the start of the nineteenth century they adopted at once the use of modern alphabet for 'S' instead. Were the alphabets in use got changed at the turn of the nineteenth century? Otherwise these eighteenth century books seem quite readable and the language understandable - not so with the seventeenth century English books!